First dive at 40 meters - Newbies recreational

[Celt]

Oh ok, you didn't have another one?

At the time the practice was to buddy breath, another bad idea best forgotten, half way up I got a breath out of his reg.
P.S. he thought he had enough air in a single 72

 

[Storker]

18m/min is no joke from 40m.

If a - let's say "moderately experienced" - diver were to attempt 18 m/min, my money would be on something between "cork" and "Polaris missile".

I think 50l/min for 2 divers in an emergency is not high at all.

15 L/min (which is not on the high side in cold water drysuit country and close to where the OP probably is, given the data we've seen in this thread) times two divers is 30. Times two for stress (standard recommendation for min gas calcs) is 60.

IMNSHO 50 is not only "not high at all". It's too low.

 

[Dody]

Yes, different training bodies use different values. BSAC once used 30bar to start the ascent, then 30bar on surface, later it moved to 50bar, now its 70bar on the surface with new divers planning their dives on thirds.

Some research for you: You have a 300bar 12Lt cylinder how much usable gas have you got? Its not a trick question because those of us who have 300bar cylinders must know the answer.

Providing that my equipment (regulator, cylinder) allow 300 bars (I currently would not exceed 230 because of the air compressibility factor), I would say that I want to surface with at least 50 bars so my safe dive plan should only account for a 250 bars air consumption, 3000 liters. Then I divide by the compressibility factor which is greater than 1,1 at 300 bars. A fair estimate would give 2500 liters.

 

[Dody]

If a - let's say "moderately experienced" - diver were to attempt 18 m/min, my money would be on something between "cork" and "Polaris missile".


15 L/min (which is not on the high side in cold water drysuit country and close to where the OP probably is, given the data we've seen in this thread) times two divers is 30. Times two for stress (standard recommendation for min gas calcs) is 60.

IMNSHO 50 is not only "not high at all". It's too low.

I don’t do cold waters or dry suit. Cold for me is 22 degrees C.

 

[David Novo]

If you have an adequate amount of gas, you do not need to ascend at 18m/min in an emergency.

For a dive to 40m and following PADI's ascent profile, I would calculate my gas reserve as follows:

Average pressure = (5+1) / 2 = 3 bar
Ascent time = 40m / 9m/min + 3 min (safety stop) + 1 min to solve issues at bottom = round to 8 min
Combined SAC = 30 l/min x 2 = 60 l/min
Reserve gas = 3 bar x 60 l/min x 8 min = 1440 litres = 120 bar (assuming a 12L steel tank - aluminium "12L" tanks just have 11.1L)
Usable gas (assuming 200 bar in the beginning) = 200 bar - 120 bar = 80 bar

This assumes direct ascent from the bottom.
If you still need to return to where you started (desirable but not mandatory as in an overhead environment), your usable gas is 40 bar each way. This means that you should turn back and start ascending at 160 bar.

P.S.: Were you taught to plan gas as shown in the spreadsheet? A 50 bar reserve is not appropriate for dives to any depth.

Providing that my equipment (regulator, cylinder) allow 300 bars (I currently would not exceed 230 because of the air compressibility factor), I would say that I want to surface with at least 50 bars so my safe dive plan should only account for a 250 bars air consumption, 3000 liters. Then I divide by the compressibility factor which is greater than 1,1 at 300 bars. A fair estimate would give 2500 liters.

50 bar at the surface is too short for a dive to 40m, as there is a considerable risk that your buddy plus your increased consumption due to stress in an emergency will amount to more than that.

 

[Dody]

I don’t do cold waters or dry suit. Cold for me is 22 degrees C.

Now, I know that at depth, it can be colder.

 

[David Novo]

Providing that my equipment (regulator, cylinder) allow 300 bars (I currently would not exceed 230 because of the air compressibility factor), I would say that I want to surface with at least 50 bars so my safe dive plan should only account for a 250 bars air consumption, 3000 liters. Then I divide by the compressibility factor which is greater than 1,1 at 300 bars. A fair estimate would give 2500 liters.

At what pressure would you turn back and at what pressure would you start to ascend?

 

[Dody]

At what pressure would you turn back and at what pressure would you start to ascend?

Return at 230 bars. Start ascent at 160 bars.

 

[Dody]

Return at 230 bars. Start ascent at 160 bars.

I am sure that you will say that 160 bars is not enough to safely ascend with a buddy having trouble.
That would mean that it is not safe to do a 40 meters dive with a 12 l tank (let alone a 80 cu ft) at 200 bars and that you definitely need a second tank.

 

[Storker]

I don’t do cold waters or dry suit. Cold for me is 22 degrees C.

and close to where the OP probably is, given the data we've seen in this thread